The present invention relates to tapered block copolymers. More particularly the present invention relates to such tapered block copolymers having improved melt flow and other properties particularly when hydrogenated.
Block copolymers, particularly block copolymers of monovinylidene aromatic monomers and conjugated dienes are well known in the art. Such polymers have found numerous uses as elastomers, as components of adhesives, and as impact modifying additives for numerous engineering thermoplastic resins. Hydrogenated block copolymers are also previously known in the art. Such hydrogenated block copolymers are frequently desired where stability and ultraviolet light degradation resistance are important. Frequently exterior applications such as impact modification of thermoplastic resins to be exposed to outdoor conditions require the use of a hydrogenated block copolymer. Numerous techniques for hydrogenation of block copolymers are known in the art and are taught for example in U.S. Pat. Nos. 3,333,024; 3,595,942; 3,868,354; 3,872,072; 3,988,504; 4,501,857; and 4,673,714.
Conventional, hydrogenated, untapered, block copolymers have been found to have an extremely high melt viscosity as measured by melt index and accordingly such hydrogenated, untapered, block copolymers have been found to be fabricated into useful objects only with great difficulty. Such difficulties in the use of hydrogenated block copolymers have been previously recognized in the art. In U.S. Pat. No. 3,507,934 an attempt to reduce the melt viscosity of such block copolymers by broadening the molecular weight distribution of the monovinylidene block is disclosed. In U.S. Pat. No. 3,700,633 a block copolymer is disclosed wherein the conjugated diene block was replaced with a completely random copolymer of a monovinylidene aromatic monomer and a conjugated diene (specifically styrene and butadiene). The resulting polymers achieved only a limited improvement in viscosity properties.
In U.S. Pat. No. 3,792,127 a further effort to reduce melt viscosity of hydrogenated block copolymers by addition of a small quantity of a random monovinylidene aromatic/conjugated diene polymer block to otherwise standard block copolymers is disclosed. The resulting polymers likewise fail to achieve significant improvement in melt viscosity properties.
U.S. Pat. No. 3,431,323 taught that triblock copolymers of styrene-butadiene-styrene should incorporate between 35 and 55 percent 1,2-vinyl functionality in the polybutadiene block to result in desirable elastomeric properties for the hydrogenated polymer. Such 1,2-vinyl functionality indicates the presence of pendant side chains or a branched structure in the resulting hydrogenated polymer.
For the teachings contained therein the foregoing U.S. patents are incorporated herein in their entirety by reference thereto.
However, it has now been discovered that if certain randomizing agents are used to produce the desired quantity of 1,2-vinyl functionality in tapered, styrene-butadiene-styrene triblock copolymers, particularly tetrahydrofuran, the taperness of the resulting polymer is adversely affected. That is, in order to produce block copolymers having reduced melt viscosity, it has not been previously possible to attain a desired 1,2-vinyl content.
It would be desirable if there were provided a hydrogenated, tapered, styrene-butadiene-styrene triblock copolymer having improved melt viscosity.
In addition, it would be desirable if there were provided a hydrogenated, tapered, styrene-butadiene-styrene triblock copolymer having improved melt viscosity wherein the 1,2-vinyl content of the polybutadiene block of the copolymer before hydrogenation is from 35 to 55 percent based on total ethylenic unsaturation in the polybutadiene block, most preferably 40-50 percent.
Finally, it would be desirable if there were provided a process for preparing tapered, styrene-butadiene-styrene triblock copolymer having both a desired range of isolated styrene content to provide improved melt viscosity and a desired range of 1,2-vinyl functionality such that the resulting polymer after hydrogenation possesses good elastomeric properties.